We have been studying the murine mammary system in which a definite preneoplastic stage has been demonstrated as a model for breast cancer. During the past five years, the metabolic and enzymatic properties of the normal mammary gland in all of its lactational states; the preneoplastic, hyperplastic alveolar nodule; and the mammary adenocarcinoma have been characterized. Our current goal is to identify means of altering the expression of a transformed genome which confers a carcinogenic potential to the cell. We will use the metabolic properties examined previously along with membrane structure, composition, enzyme content, and function to predict the carcinogenic consequences of any observed change.